Laser marking additives are known and are described in the prior art.
Laser marking additives can be added to various matrix polymers to be able to leave a mark on an article made from the matrix polymer by after treating the article with laser light.
Laser marking additives preferably are employed in very small amounts and have no or a very light own color.
The use of laser marking additives based on antimony and bismuth are well known.
Such laser marking additives are described, for example in WO2004050766, WO2009003976, WO2011050934 and WO2014060099.
A disadvantage of laser marking additives based on bismuth is that they are not suitable in all types of plastics. In certain matrix polymers the bismuth compound shows severe discoloration when high processing temperatures, for example higher than 220° C., are used. Bi2O3 can, for example, not be used as an additive in polyimide, since during processing an exothermic reaction takes place resulting in a dark product.
WO2004050766 describes a laser light absorbing additive comprising particles that contain at least a first polymer with a first functional group and 0-95 wt. % of an absorber. The absorber can, for example, comprise bismuth. The particles are microspheres, containing a core and a shell. The microspheres described in WO2004050766 have proven to provide a better resolution and contrast by laser irradiation than the composition disclosed in other prior art. However, the contrast obtained with these microspheres, especially at (very) high marking speeds is still relatively low. Moreover, high contrast is mainly obtained while marking with high laser pulse energies. Marking should therefore preferably be performed in focus of the laser beam which limits the marking area per pulse and hence increases marking time.
WO2011050934 describes a laser marking additive composition comprising a bismuth containing compound and a functionalized polymer having 0.01 to 50 wt. % functional groups. The laser marking additive is added in small amounts (up to 2 wt. %) to a matrix polymer to obtain a lasermarkable composition. The disadvantage of this laser marking additive is that the functionalized polymer does not contribute to the color forming process and therefore reduces the marking performance with respect to especially marking speed. Also because of this, applying this laser marking additive in non or insufficient laser-markable polymers such as polyolefins, will result in very poor marking performance with respect to both marking contrast and speed. Moreover, upon processing above the generally low melting point (Tm<160° C.) of the functionalized polymer in certain matrix polymers, the laser marking additive will show discoloration due to migration of the bismuth compound to the matrix polymer.
WO2014060099 describes microspheres and their use as laser absorbing additives. The microspheres have a core/shell structure and the core comprises at least one bismuth compound. The disadvantage of the microspheres according to WO2014060099 is that at least two carriers are needed, one for the core and one for the shell, before the microsphere can be dispersed in the matrix polymer. Furthermore, two processing steps are required to obtain the laser marking additive which results in additional costs.
WO2000/78554 describes laser marking compositions. As marking pigment TiO2 is used, while as a fixing material a bismuth trioxide can be applied. The TiO2 and bismuth oxide are applied as a layer on a substrate, which is subsequently cured. The composition that is applied on the substrates comprises a propylene glycol as a solvent. Upon application on the substrate and curing of the composition, the solvent is evaporated. After curing the laser marking is performed.
CN105085944 describes a laser marking thermoplastic polyurethane special material which is prepared by crushing a metaloxide, dissolving it in hydrochloric acid, adding an alcohol under UV irradiation, heating and cooling while adding sodium chloride and a further material having a function as a surfactant and a polyurethane matrix resin.